The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
This invention relates to methods for controlling lost circulation in subterranean wells, in particular, fluid compositions and methods for operations during which the fluid compositions are pumped into a wellbore, enter voids in the subterranean-well formation through which wellbore fluids escape, and form a seal that limits further egress of wellbore fluid from the wellbore.
During construction of a subterranean well, drilling and cementing operations are performed that involve circulating fluids in and out of the well. The fluids exert hydrostatic and pumping pressure against the subterranean rock formations, and may induce a condition known as lost circulation. Lost circulation is the total or partial loss of drilling fluids or cement slurries into highly permeable zones, cavernous formations and fractures or voids. Such openings may be naturally occurring or induced by pressure exerted during pumping operations. Lost circulation should not be confused with fluid loss, which is a filtration process wherein the liquid phase of a drilling fluid or cement slurry escapes into the formation, leaving the solid components behind.
Lost circulation can be an expensive and time consuming problem. During drilling, this loss may vary from a gradual lowering of the mud level in the pits to a complete loss of returns. Lost circulation may also pose a safety hazard, leading to well-control problems and environmental incidents. During cementing, lost circulation may severely compromise the quality of the cement job, reducing annular coverage, leaving casing exposed to corrosive downhole fluids, and failing to provide adequate zonal isolation. Lost circulation may also be a problem encountered during well-completion and workover operations, potentially causing formation damage, lost reserves and even loss of the well.
Lost-circulation solutions may be classified into three principal categories: bridging agents, surface-mixed systems and downhole-mixed systems. Bridging agents, also known as lost-circulation materials (LCMs), are solids of various sizes and shapes (e.g., granular, lamellar, fibrous and mixtures thereof). They are generally chosen according to the size of the voids or cracks in the subterranean formation (if known) and, as fluid escapes into the formation, congregate and form a barrier that minimizes or stops further fluid flow. Surface-mixed systems are generally fluids composed of a hydraulic cement slurry or a polymer solution that enters voids in the subterranean formation, sets or thickens, and forms a seal that minimizes or stops further fluid flow. Downhole-mixed systems generally consist of two or more fluids that, upon making contact in the wellbore or the lost-circulation zone, form a viscous plug or a precipitate that seals the zone.
A thorough overview of LCMs, surface-mixed systems and downhole-mixed systems, including guidelines for choosing the appropriate solution for a given situation, is presented in the following reference: Daccord G, Craster B, Ladva H, Jones T G J and Manescu G: “Cement-Formation Interactions,” in Nelson E and Guillot D (eds.): Well Cementing—2nd Edition, Houston: Schlumberger (2006): 202-219.
Swellable materials may be employed as bridging agents, either alone or in a mixture of different bridging agents. The swellable materials increase in size and/or form gels upon mixing with aqueous or hydrocarbon-base fluids, depending on their chemistries. For example, this concept was described by Klaas et al. in U.S. Pat. No. 2,935,472 and, more recently, by Creel et al. in U.S. Patent Application 2006/0086501 A1.Broad varieties of swellable polymers that are suitable for curing lost circulation are revealed by McKinley et al. in U.S. Pat. No. 4,526,240.
Swellable polymers suffer from a fundamental problem in that their ability to swell is limited by the presence of soluble salts in the carrier fluid that increase ionic strength. For example, exposing swellable polymers to formation waters that contain high concentrations of electrolytes (e.g., Na+ and Ca2+) severely limits the degree to which swelling occurs, and reduces the polymers' ability to address lost circulation. Therefore, there is a need for a swellable-polymer system that is relatively independent of fluid chemistry, and may swell or expand under a broad range of downhole conditions.